Vacuum Reserve Tank; Vacuum G A U G E; Vacu U M Pump - GMC 4000 Series Service Manual

Sec. 5A
Page 184
HYDRAULIC BRAKES
A vacuum reserve tank is used as standard
equipment on "S" 5500 and 6500 Models and as op­
tional equipment on all other models using vacuum
assisted hydraulic brakes. This tank has a mini­
mum capacity of 1,000 cubic inches and is installed
in the vacuum line between the vacuum check valve
and the power cylinder.
On conventional cab models, the tank is in­
stalled immediately below the left-hand door, be­
The vacuum gauges used on vehicles covered
by this manual all operate on the same principle
and are factory set and sealed units which are not
adjustable or repairable. If a gauge fails to oper­
ate or operates improperly, it must be replaced.
All vacuum gauges are located in the instrument
panel.
A vacuum gauge is standard equipment on
some models and optional on others.
If a vapcuum gauge is suspected of operating
improperly, it may be checked by comparing read­
ings with a test gauge which is known to be accur­
ate. Observe readings on vehicle gauge at engine
idle speed and at various, specific engine rpm up
GENERAL DESCRIPTION
The vacuum pump is a rotary, sliding vane
type pump in which the rotor is eccentrically lo­
cated with respect to the housing. The pump is
dependent upon oil as a means for lubrication of
the bearings and also for providing a seal.
The vacuum pump is operated directly by a
belt and pulley drive from the engine and operates
at all times while the engine is running. The lubri­
cation of the pump is accomplished by connection
with the oil pressure system of the engine.
DISCHARGE OF OIL AND AIR
The oil plus the air which enters through the
vacuum port, is discharged through the housing
just below the point of minimum rotor to housing
clearance. The oil and air leaves the pump through
the opening on the pump base which is connected
to the engine crankcase by a discharging hose.
DRIVE PULLEY
Maximum pump speed, for good life expect­
ancy, is 4,000 rpm. If it is necessary to replace
G M C S E R V I C E M A N U A L
V A CU U M RESERVE TANK
hind the cab step.
hind the engine, in front of the rear axle, and be­
tween frame side rails.
side of the left-hand side rail just to the rear of the
batteries.
connecting tank to vehicle.
V A C U U M GAUG E
to maximum. Install test gauge in convenient loca­
tion in vacuum line and observe readings on test
gauge at same, specific engine rpm. The manu­
facturer's specifications permit a variation of - 1-
inch of mercury at 5 inches and - 2 inches of mer­
cury at 20 inches. Any variation beyond these
lim its is an indication that the vehicle vacuum
gauge should be replaced. Before condemning a
gauge which does not register, or registers im ­
properly, make certain that all vacuum lines in the
system are free of dirt and/or kinks and that all
connections are tight. System leakage can result
in registration on gauge which is not normal. Also,
on models with diesel engine, make certain that
vacuum pump is properly installed and operating.

VACU U M PUMP

the drive pulley, be certain that correct pulley is
used. If pulley diameter is too large the pump ca­
pacity will be decreased; if the pulley is too small
pump speed will exceed maximum recommended
and premature pump failure will result. Refer to
Parts Catalogue for proper replacement pulley.
the vacuum pump while it is operating.
LUBRICATION
minutes, check the oil and air discharge line near
the pump discharge port. If the line is not warming
up, stop the pump and check for restrictions in the
oil supply line.
VACUUM
al operating speed for several minutes so as to
warm the oil circulating through the pump. Check
the vacuum gauge. The pump should maintain at
On tilt cab models, the tank is installed be­
On M S" models the tank is installed on the out­
To replace tank, disconnect lines and brackets
SYSTEM TESTS
The following tests or checks can be made on
After the pump has been running for a few
Let the pump and engine operate at their norm­